Mobile communication system uses a technique of converting a baseband signal or an Intermediate Frequency (IF) signal to an RF signal, or converting an RF signal to a baseband signal or an IF signal using an IQ mixer.
However, in an actual IQ mixer, carrier leakage and IQ imbalance occur. The carrier leakage occurs because not only product of an input signal and an IQ LOcal frequency (LO) signal transferred from a local oscillator is transferred to an output terminal of the IQ mixer, but also an IQ LO signal leaks to the output terminal of the IQ mixer. The IQ imbalance includes gain imbalance that occurs because the sizes of an in-phase signal and a quadrature signal transferred from the local oscillator to the IQ mixer are not the same, and phase imbalance that occurs because an in-phase signal and a quadrature signal do not have a phase difference of 90°. When the carrier leakage and the IQ imbalance occur, an undesired noise component exists in an output of the IQ mixer to deteriorate a Signal-to-Noise Ratio (SNR), so that a bit error rate of a mobile communication system increases and so link performance deteriorates.
A transmitter of the conventional mobile communication system compensates for gain imbalance and phase imbalance of an IQ channel by compensating for an imbalance component at a baseband modem in order to remove an image frequency component.
In a method for removing an image frequency component, since compensation of an imbalance component of an IQ channel is performed inside a baseband modem, when a frequency in a band equal to or higher than 2 GHz is used as a carrier frequency, imbalance of Integrated Circuit (IC) itself, more particularly, phase imbalance becomes inevitably serious, so that a burden which the baseband modem should process increases.
In addition, unlike the above description, a transmitter of the conventional mobile communication system may cancel an image frequency component using a compensation circuit inside an RFIC.
For this purpose, an output terminal of an IQ modulator monitors an image frequency component and compensates for IQ phase imbalance by controlling the phases of buffers existing on an I-path and a Q-path of an LO until the image frequency component generated by the IQ phase imbalance reduces below a predetermined level.
The above-described method may compensate for IQ phase imbalance inside an RFIC but cannot compensate for a gain imbalance component.